The assignee of the present invention manufactures and deploys spacecraft for commercial, defense and scientific missions. On board propulsion systems of such spacecraft are frequently required to perform orbit raising (or transfer). For example, there is frequently a requirement for commercial spacecraft to perform orbit raising from a launch vehicle transfer orbit (or “parking orbit”) to, for example, a geosynchronous orbit. Following separation from the launch vehicle, the spacecraft then performs transfer orbit operations to transfer the spacecraft from the parking orbit to the geosynchronous orbit. Certain aspects of orbit raising or described in U.S. Pat. No. 8,763,957, assigned to the assignee of the present invention and hereby incorporated by reference in its entirety into the present application.
Spacecraft propulsion systems generally include thrusters, which may be broadly categorized as either “chemical” or “electric” based on the respective primary energy source for performing these transfer orbit operations.
Chemical thrusters, for example bipropellant thrusters, deliver thrust by converting chemical energy stored in the propellant to kinetic energy delivered to combustion products of the chemical propellant, e.g., a fuel such as monomethyl hydrazine and an oxidizer such as dinitrogen tetroxide. Whether the propellant is a monopropellant or a bipropellant, chemical thrusters deliver thrust by converting chemical energy stored in the propellant to kinetic energy delivered to combustion products of the chemical propellant. Chemical thrusters, as the term is used herein, and in the claims, also include electrothermal thrusters such as arcjets, described for example in U.S. Pat. Nos. 5,485,721 and 5,819,526, that are configured to use electrical energy to increase the temperature, and, therefore, the velocity of the combustion products of chemical propellants.
In contrast, an electric thruster, as the term is used herein, and in the claims, converts electrical energy to propellant kinetic energy substantially without regard to any chemical energy the propellant may possess. For example, an electric thruster may operate by ionizing and accelerating a gaseous propellant, where the propellant is a noble gas of a heavy element, such as xenon or argon. Irrespective of the selected propellant, a negligible amount of thrust results from energy chemically stored in the propellant. The term electric thruster, as used herein and in the claims, encompasses an electrostatic thruster, an electromagnetic thruster, a Hall effect thruster, a wakefield accelerator, and a traveling wave accelerator, for example.
Chemical thrusters suitable for spacecraft propulsion systems may deliver relatively high thrust of 10-1000 newtons, for example, substantially irrespective of spacecraft power limitations, but such thrusters are generally incapable of operating at a specific impulse (Isp) higher than 500 seconds. Electric thrusters may operate at an Isp of 1000-4000 seconds, but spacecraft power constraints, at least, practically constrain thrust levels to well less than one newton.
As disclosed in U.S. Pat. No. 9,145,216, assigned to the assignee of the present invention and hereby incorporated by reference in its entirety into the present application, during the course of a typical spacecraft mission there are times that a high thrust, low power thruster is desirable; at other times, however, a low thrust, high Isp thruster is more advantageous. As a result, it is known, as illustrated in FIG. 1, to provide both chemical and electric thrusters on board a single spacecraft, each thruster assigned to a propulsion subsystem having its own dedicated propellants and its own dedicated propellant and pressurant supply arrangements.
As illustrated in FIG. 1, a chemical propulsion subsystem 110 may include any number of chemical thrusters 116 manifolded by way of a control module 115 with fuel tank 113 and oxidizer tank 114. The fuel tank 113 and the oxidizer tank 114 may each be loaded with a desired quantity of liquid propellant, and include an ullage volume, gaseous pressure of which may be regulated by a pressure control module 112. For example the pressure control module 112 may include one or more pressure regulators. Helium (He) stored in pressurant tanks 111 at a high pressure may be reduced in pressure by the pressure control module 112 and delivered to the fuel tank 113 and the oxidizer tank 114.
An electric propulsion subsystem 120 may include any number of electric thrusters 126 manifolded by way of a propellant management assembly (PMA) 122 with propellant tanks 121. Propellant such as xenon (Xe) stored in tanks 121 at a high pressure may be reduced in pressure by the PMA 122 and delivered to the electric thrusters 126.
The arrangement illustrated in FIG. 1 may be separately optimized for particular mission requirements. For example, each of the He tanks 111 and Xe tanks 121 may be separately sized based on a respective anticipated amount of chemical propulsion subsystem operation and electric propulsion subsystem operation.